Exhaust gas generated by combustion of fossil fuels in furnaces, ovens, and engines contain, for example, nitrogen oxides (NOX), unburned hydrocarbons (HC), and carbon monoxide (CO), which are undesirable pollutants. Vehicles, e.g., diesel vehicles, utilize various pollution-control after treatment devices (such as a NOX absorber(s) and/or Selective Catalytic Reduction (SCR) catalyst(s)), to reduce NOX. For diesel vehicles using SCR catalysts, NOX reduction can be accomplished by using ammonia (NH3) gas. However, the presence of NH3 can also interfere with various types of NOX sensors, thereby reducing their accuracy. In order for SCR catalysts to work efficiently and to avoid pollution breakthroughs, more effective control systems are needed. The development of more effective control systems requires commercial NOX sensors with the improved accuracy and sensitivity to the various NOX constituent species with reduced susceptibility to NH3 cross-interference.
For example, existing NOX sensing materials having the chemical composition of the general form ([A][B])204 or [A][B]03, which include stoichiometric amounts of A, where A is metal elements capable of +3 valence state, and B, where B is Fe or Cr, are sensitive to NO and NO2. They are used as electrode materials in electrochemical devices for NOX sensing by virtue of the fact that they generate an electromotive force (emf) when exposed to NOX, the magnitude and polarity of which may be characterized using the non-equilibrium Nernst Equation. However, generally, they are sensitive to cross-interference with NH3 which tends to limit their usefulness to applications where NH3 is not a concern. In addition, their NOX electromotive force (emf) outputs are not large, resulting in non-optimal signal-to-noise ratios. As such, while useful in some applications, these sensors are not generally suitable for applications that require sensors that are insensitive to cross-interference with NH3 or relatively larger NOX electromotive force (emf) outputs (i.e., improved signal-to-noise performance) or both.
Thus, cost effective NOX sensors having reduced sensitivity to cross-interference from NH3, or relatively high emf outputs or both, that can reliably sense NOX under exhaust gas conditions would be desirable for use in various NOX control systems.